The noise generated by rotating machines and their equipment may have high acoustic power levels that can attain 130 dB(A) (weighted decibel A) to 160 dB(A) and which can spread to neighboring equipment. For example, a protective enclosure, air inlet ducts, or air outlet ducts of a gas turbine may be the preferred paths for spreading acoustic waves.
Thus, a reduction in the noise radiated by the equipment is possible with the use of panels of an appropriate structure. In general, the structure of panels may have at least two walls, one external sheet and one internal sheet, with at least one layer of acoustic insulation material placed between the two sheets, and stiffeners or binding elements between the panels in which the elastic dampener methods may dissipate the transmission of vibrations through the structure.
The acoustic phenomenon in the panel structures may be classified into three types:                Reflection: a part of the acoustic wave is sent to an internal part of the equipment.        Absorption: dissipation of energy from a wave in the walls and in the materials (or air) placed between the walls.        Transmission: emission of energy by vibration of the entire structure towards the exterior of the equipment.        
As described in the document EP 1 657 374, the acoustic panels are made of metal. The panel design is based on the theory that mass is the main factor providing for acoustic reduction. This is based on the important fact that the more the mass increases the more the noise may be reduced. Thus, the acoustic panels including the metal sheets may provide a noise reduction by the effect of mass. A porous material also provides a reduction through the effect of viscosity and friction. Dampers dispel the vibrations and the acoustic energy between the external and internal walls, as these elements are typically composed of an elastic or flexible material such as rubber.
Absorption of acoustic waves in a panel structure according to the documents EP 2 017 826 and FR 2 356 820 may be executed by Helmholtz resonators, particularly for applications in an air inlet close to a gas turbine compressor. Further, as proposed in the document U.S. Pat. No. 4,084,367, Helmholtz resonators may be used for the acoustic absorption of frequency waves in the range of 250 and 2000 Hz. In the document FR 2 356 820, resonator chambers of various volumes or lengths ensure the stifling of the noise over a wide range of frequencies.
As far as the dampening of vibrations is concerned, document U.S. Pat. No. 5,907,932 proposes two horizontal connection elements between two sheet panels and one dampening element between the connecting elements absorb the vibrations crossing the structure. Document U.S. Pat. No. 7,467,687 proposes the use of two elastic elements on either side of a connecting element and placed at an interface between the element and each sheet. The vibration dampening elements are typically made of rubber or elastic material. Thus, the acoustic absorption is carried out either by an absorbent material or by resonators. Reduction of the acoustic transmission phenomenon through the structure is realized by the vibration dampers with elements typically of an elastic material.
The transmission and spreading of noise through structural walls, including through the insulating material, may cause the excitation of resonance modes of the structure through solid vibration, thus creating radiation of noise by the surface of the external wall. This phenomenon may be reduced by the multiplication of vibration damper points in all of the elements connecting the structure and the panels. This multiplication tends to increase the number of elements of the structure and may extend the assembly time of the panels.
In fact, during maintenance operations, it is sometimes necessary to carry out the removal and reassembly of the acoustic structure around the equipment, without it being guaranteed that the resulting acoustic performances may be equivalent to those provided initially. In particular, the elastic damper elements, cleared of vibrations, may age due to the high temperatures and vibration levels characteristic in the operation of the rotating machines.